linux-kernel - Re: [PATCH v4] blk-cgroup: Replace u64 sync with spinlock for iostat

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Message-ID: <4f06abd438009266997bd41901c05c9139e19472.camel@mediatek.com>
Date: Fri, 9 Aug 2024 07:50:41 +0000
From: Boy Wu (吳勃誼) <Boy.Wu@...iatek.com>
To: "tj@...nel.org" <tj@...nel.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"boris@....io" <boris@....io>, "linux-block@...r.kernel.org"
	<linux-block@...r.kernel.org>, "linux-mediatek@...ts.infradead.org"
	<linux-mediatek@...ts.infradead.org>, "cgroups@...r.kernel.org"
	<cgroups@...r.kernel.org>, "axboe@...nel.dk" <axboe@...nel.dk>,
	Iverlin Wang (王苳霖) <Iverlin.Wang@...iatek.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "matthias.bgg@...il.com"
	<matthias.bgg@...il.com>, "angelogioacchino.delregno@...labora.com"
	<angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH v4] blk-cgroup: Replace u64 sync with spinlock for iostat

On Tue, 2024-07-30 at 09:49 -1000, tj@...nel.org wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  Hello, Boy.
> 
> On Fri, Jul 26, 2024 at 03:43:27AM +0000, Boy Wu (吳勃誼) wrote:
> ...
> > The use of a spinlock with u64 sync is suggested in
> > include/linux/u64_stats_sync.h:33.
> > 
> >  * Usage :
> >  *
> >  * Stats producer (writer) should use following template granted it
> > already got
> >  * an exclusive access to counters (a lock is already taken, or per
> cpu
> >  * data is used [in a non preemptable context])
> >  *
> >  *   spin_lock_bh(...) or other synchronization to get exclusive
> access
> >  *   ...
> >  *   u64_stats_update_begin(&stats->syncp);
> >  *   u64_stats_add(&stats->bytes64, len); // non atomic operation
> >  *   u64_stats_inc(&stats->packets64);    // non atomic operation
> >  *   u64_stats_update_end(&stats->syncp);
> > 
> > Is this a incorrect statment?
> 
> That's not incorrect and it'd make sense if we really want to use
> u64_sync -
> e.g. the reader is hot path. Here, just a spinlock would be simpler
> and do
> fine.
> 
> Thanks.
> 
> -- 
> tejun


u64_sync with spin lock has the benefit of locking only when writing
iostat, but replacing u64_sync with spin lock will lock not only when
writing iostat but also when reading iostat. Does it have enough
benefit to replace u64_sync and add the cost of locking when reading
iostat?

--
Boy.Wu